Permanent magnets of substantially R-TM-B are widely used because they are inexpensive and have high magnetic properties. Because R-TM-B magnets have excellent magnetic properties and high mechanical strength, and are less brittle, they can withstand internal stress due to sintering shrinkage. Accordingly, they can easily formed into radially anisotropic magnets and multi-polar-anisotropic ring magnets, resulting in motors with high power and small sizes.
When radially anisotropic ring magnets are used for rotors for brushless motors, ring magnets are fixed to rotor yokes mostly by adhesives or injection-molded resins. Because of recent demand for high quality and reliability for motors, etc., various proposals have been made to prevent the relative rotation of ring magnets and rotor yokes due to reduced bonding strength.
JP 10-201152 A discloses a permanent magnet rotor comprising a polar-anisotropic ring magnet retaining projections formed on the inner peripheral surface when sintered, and a rotor yoke having recesses at positions corresponding to the projections on the inner peripheral surface, the rotor yoke being inserted into the polar-anisotropic ring magnet such that the projections are aligned with the recesses, and their bonding strength being increased by a filling member. This reference describes that by utilizing projections and recesses without working as described above, the fixing strength of an annular permanent magnet to a rotor shaft can be increased. However, because the use of projections and recesses on the inner peripheral surface without working provides large unevenness in the inner diameter, there should be a large gap to some extent between the outer diameter of the rotor yoke inserted and the inner diameter of the magnet, so that the magnet cannot sufficiently exhibit magnetic properties.
JP 2005-304178 A discloses a technology for preventing the relative rotation of a ring magnet to a rotor yoke by grinding a sintering ring magnet to have a desired inner diameter with axial recesses on the inner surface at positions corresponding to magnetic poles, grinding a rotor yoke to have axial projections on the outer peripheral surface at positions corresponding to the recesses, and engaging the sintering ring magnet around the rotor yoke. JP 2005-304178 A describes that axial recesses can be formed by removing by grinding projections from a rough inner peripheral surface of the polar anisotropic ring magnet formed by anisotropic sintering shrinkage of magnet powder. Because projections are ground off from the inner peripheral surface of the magnet, a constant gap can be obtained between the rotor yoke and the inner surface of the magnet. However, because roughness caused by an oriented magnetic field in the polar-anisotropic magnet is utilized, only polar-anisotropic magnets can be used, but radially anisotropic magnets cannot be used.
Although the polar anisotropic ring magnets of JP 10-201152 A, JP 2005-304178 A, etc. have high peaks in surface magnetic flux density waveforms after magnetization, the waveforms being almost sinusoidal, magnet powder is oriented to achieve a final waveform pattern during molding, so that uneven powder supply tends to cause unevenness in magnetic properties because of problems in molding, such as the disturbance of a waveform, etc. Also, polar orientation necessitates a thick magnet. On the other hand, radially anisotropic magnets can have different waveform patterns depending on magnetization methods, increasing the degree of freedom of designing motors. Accordingly, they are used for various motors. Against this backdrop, inexpensive, radially anisotropic ring magnets that can be surely locked to rotors have been desired.
Rotors comprising radially anisotropic ring magnets are widely used for motors. However, because magnetization does not easily provide radially anisotropic ring magnets with sinusoidal surface magnetic flux density waveforms, resulting in trapezoidal waveforms (rapid change of magnetic flux densities at ends of magnetic poles), the radially anisotropic ring magnets tend to have poor cogging characteristics. Because large cogging torque generates noise and vibration in motors, the reduction of cogging torque is one of the important objects in designing motors.
JP 3-265102 A discloses a radially anisotropic ring magnet having one or more axial grooves on the inner or outer peripheral surface. It describes that grooves absorbs tensile stress and compression stress generated in a cooling process after sintering, thereby preventing cracking. It describes that grooves on the inner or outer peripheral surface are formed by a non-magnetic die having axial ridges. However, the method of JP 3-265102 A provides large difference in a magnetic flux density between the non-magnetic ridges and the magnet powder in a magnetic field during molding, resulting in disturbance in the orientation of the magnet powder near the grooves of the magnet, and thus deformation and cracking during sintering. When the magnet is used for a motor rotor, the motor has large cogging torque because of disturbance in a surface magnetic flux density waveform after magnetization.
JP 2005-79423 A discloses a die for producing a radially anisotropic ring magnet having roughness on the peripheral surface, the die having a cylindrical cavity defined by a ferromagnetic body, and a peripheral surface of the cylindrical cavity being provided with axial non-magnetic projections and recesses, thereby reducing disturbance in the orientation of magnet powder during molding. Because this die is designed based on a magnetic field orientation when there is no magnet powder in the cavity, there is large magnetic flux density difference between the non-magnetic projections and recesses and the magnet powder in an orientated magnetic field during molding. Accordingly, there is large disturbance in the orientation of magnet powder near the recesses particularly in the case of a short roughness period, resulting in deformation and cracking during sintering. When a ring magnet with largely disturbed orientation of magnet powder is used for a motor rotor, there is disturbance in a surface magnetic flux density waveform after magnetization, resulting in large cogging torque.
To suppress sintering deformation in anisotropy sintering magnets to improve their magnetic properties, JP 9-45568 A discloses a compression-molding method in a magnetic field using a die comprising a member having saturation magnetization 4πIs of 0.5-1.2 T on at least a cavity-facing surface. However, the method of JP 9-45568 A molds magnets with flat surfaces such as rectangular solids, failing to disclose a technology for providing radially oriented ring magnets with partial roughness on the inner peripheral surfaces, such that deformation is suppressed near the roughness. In addition, it does not describe a magnetic die member having 4πIs of more than 1.2 T.